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The Chemistry of Life: The Plastic in Cars

Even if cars soon start running entirely on electricity or hydrogen, they'll still need 100 gallons or more of oil to make their plastic parts, such as seats, dashboards, bumpers, and engine components. And some day that plastic may be recycled back into fuel.

Cars of old were mostly steel, but the use of lightweight alternatives has dramatically increased in the last couple of decades. Whereas almost no plastic could be found on a car from the 1950s, today's automobiles have more than 260 pounds (120 kilograms) of plastic on board, according to the Transportation Energy Data Book.

"It is expected that high oil prices and strict CO2 standards will accelerate the growth [in plastic use]," says Aafko Schanssema from PlasticsEurope, a plastic industry group based in Belgium.

Plastics improve fuel economy by reducing weight, but they also require petroleum as a raw ingredient.

"Plastics are in fact solidified oil," Schanssema explained.

Although different plastics have different recipes, it takes roughly 0.4 gallons of crude oil to make 1 pound of plastic. Globally, around 8 percent of the oil that comes out of the ground is used to make plastic.

The average car is a mix of materials: glass windows, rubber tires, lead batteries, copper wires, as well as traces of zinc, magnesium, tin, platinum and cobalt.

However, steel is still the single most important material in cars. It is strong, durable and malleable. On the flip side, though, it is relatively heavy. For this reason, car manufacturers have been trimming down on its use.

Besides reducing weight, plastics help to streamline the shape of vehicles, improve the performance of tires and increase the safety of windshields and fuel tanks.

Still, there are ideas for making plastics more sustainable. One way might be to use bio-degradable plastics, or ones that come from renewable resources, such as corn or sugarcane.

Another option is to recover the energy from discarded plastic parts. The company Plas2fuel, based in Washington state, can make a gallon of oil from melting down 8 pounds of plastic. In March, this process was used by Oregon-based Agri-Plas to turn plastic waste into 8,200 gallons of oil.

Moisture conditioning

Processors and end users who use nylon have become very familiar with the effects that water absorption has on that material. In applications where high loads are generated, such as in snapfit assemblies, nylon that is still close to its dry-as-molded state may exhibit brittle failure, and we have learned that this failure mode can be mitigated by conditioning the parts to bring them up to their equilibrium moisture content. This frequently solves problems with the assembly process.

The moisture conditioning process takes many forms. Some simply pour a prescribed amount of water into molded parts contained in a moisture-proof package such as a polybag. Others prefer placing saturated paper towels into the package with the nylon parts and allowing the water to migrate out of the paper and into the nylon. Some go as far as boiling the parts. This not only increases the moisture uptake rate, but also ensures that the moisture is absorbed more uniformly throughout the wall of the part.

This experience contradicts a lot of the data published by material suppliers showing the conditioned moisture content at 2.5%. But much of this early work was performed using accelerated techniques that had a tendency to introduce more moisture into the polymer than it could hold in the long term. Field experience shows that values of 1.5% for an unfilled material are much closer to the norm.

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